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Surface Acoustic Wave (SAW) sensors : Application to
the detection of chemical compounds in the gaseous
state
Ioannis Nikolaou
To cite this version:
Ioannis Nikolaou. Properties of graphene by chemical recognition on Surface Acoustic Wave (SAW) sensors : Application to the detection of chemical compounds in the gaseous state. Electronics. Uni-versité de Bordeaux, 2019. English. �NNT : 2019BORD0379�. �tel-02930977�
THÈSE
présentée à
L'UNIVERSITÉ BORDEAUX
ÉCOLE DOCTORALE DES SCIENCES PHYSIQUES ET DE L’INGÉNIEUR
par
Ioannis Nikolaou
POUR OBTENIR LE GRADE DE
DOCTEUR
SPÉCIALITÉ : ÉLECTRONIQUE
Étude des propriétés de reconnaissance chimique du
graphène par ondes acoustiques de surface : Application à la
détection de composés chimiques à l’état gazeux
Soutenue le 17 décembre 2019
Devant la commission d'examen formée de:
Dr. Anne-Claire SALAÜN, Maitre de conférences HDR, Université de Rennes, France Rapporteur Prof. Philippe MENINI, Professeur des universités, Université de Toulouse, France Rapporteur Dr. Maria Carmen HORRILLO GÜEMES, Investigador Científico, CSIC, Madrid, Espagne Examinatrice Prof. Corinne DEJOUS, Professeur des universités, Bordeaux INP, France Présidente Dr. Ollivier TAMARIN, Maitre de conférences, Université de Guyane, France Invité
Dr. Hamida HALLIL, Maitre de conférences, Université de Bordeaux, France Co-directrice de thèse Prof. Dominique REBIÈRE, Professeur des universités, Université de Bordeaux, France Directeur de thèse
iii
THESIS
submitted to the
UNIVERSITY OF BORDEAUX
DOCTORAL SCHOOL OF PHYSICAL SCIENCES AND ENGINEERING
by
Ioannis Nikolaou
in partial fulfillment for the Degree of
DOCTOR OF PHILOSOPHY
in ELECTRONICS
Properties of graphene by chemical recognition on Surface
Acoustic Wave (SAW) sensors: Application to the detection of
chemical compounds in the gaseous state
Defense date: 17 December 2019 Review committee:
Prof. Anne-Claire SALAÜN, Assoc. Prof., University of Rennes, France Reviewer Prof. Philippe MENINI, Full Professor, University of Toulouse – Paul Sabatier, France Reviewer
Dr. Maria Carmen HORRILLO GÜEMES, Full Researcher, CSIC, Madrid, Spain Examiner Prof. Corinne DEJOUS, Full Professor, Bordeaux INP, France President Dr. Ollivier TAMARIN, Assoc. Prof., University of French Guiana, France Invited Dr. Hamida HALLIL, Assoc. Prof., University of Bordeaux, France Thesis Co-Director Prof. Dominique REBIÈRE, Full Professor, University of Bordeaux, France Thesis Director
v
In the name of the best within you, do not sacrifice this world to those who are its worst. In the name of the values that keep you alive, do not let your vision of man be distorted by the ugly, the cowardly, the mindless in those who have never achieved his title. Do not lose your knowledge that man's proper estate is an upright posture, an intransigent mind and a step that travels unlimited roads. Do not let your fire go out, spark by irreplaceable spark, in the hopeless swamps of the approximate, the not-quite, the not-yet, the not-at-all. Do not let the hero in your soul perish, in lonely frustration for the life you deserved, but have never been able to reach. Check your road and the nature of your battle. The world you desired can be won, it exists, it is real, it is possible, it's yours.
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ix
Acknowledgments
« Once, I was asked, how do I see myself in the future and how do I want other people to look at me. My answer to that was that I want to actually inspire the change in people; motivate them towards their best self. Following a career in academia it might not be an easy task but it would end up putting me in a position that I could actually set and realize severe goal orientations. To my perspective, in order to be able to inspire other people you have to be able to feel inspired within yourself, which usually comes by thinking of the long-term outcomes. Generally, task orientations are regarded as more adaptive than ego orientations. Task orientation is a value that every scientific goal can be related to, especially when it comes to selection of challenging tasks, effective study strategies, positive attitudes toward learning, and positive emotions, whereas quite often ego orientation is associated with selection of easier tasks, trivial learning strategies, concern for social status and thoughts of escape and behavioral withdrawal when difficulties are encountered. My ‘unquenchable fire’ for knowledge is inextricably linked with an insatiable curiosity about science and actually helps viewing an academic/research achievement, such as a Ph.D. with more fun. Thus, following my only true passion for Physics and Engineering, I started a Ph.D. in Bordeaux at October of 2013. I would like to acknowledge Prof. Dominique Rebière and Associate Prof. Hamida Hallil at IMS laboratory (CNRS UMR 5218) for giving me the opportunity to work with such an interesting and promising subject; that of graphene-based novel microsystems. Throughout the thesis I had the rare chance to approach the field of surface acoustic wave sensors from various different aspects, which was extremely fruitful for acquiring a more solid understanding, especially from the theoretical and experimental perspectives. Furthermore, I would like to thank Prof. Corinne Dejous, for the countless scientific discussions that we had over time, especially on the properties of the Love wave devices. Without her detail-oriented advices and solid scientific expertise, this thesis would not come to exist. Our discussions didn’t only help me evolve as a scientist, but also as an individual, and I am grateful for that. I was truly honored to work with her. Of course, at this point it would be a crime if I wouldn't acknowledge Dr. George Deligeorgis. The discussions that we had during my thesis were crucial to solidify the results presented herein. Thanks to him, I also had the rare opportunity to prove several concepts regarding the carbon-based materials between different laboratories (France, Spain and Greece), since he initiated the collaboration with a famous and well- known laboratory in Spain (Instituto Universitario de Tecnología Química, Universidad Politécnica de Valencia (CSIC-UPV). During his staying in France (Toulouse / LAAS - CNRS), he managed to provide guidance on my research, thus highlighting how important is the repeatability in the research community, especially after our first-year results. Furthermore, he initiated a novel collaboration with FORTH–IESL, Microelectronics Research Group in Greece, due to his establishment as a researcher. His rigorous corrections on the material-based premises (graphene) and his enormous experience on the process development concepts (deposition aspects of carbon-based materials) were important lessons for me and my future as a researcher. Additionally, I would like to thank also Prof. Hermenegildo Garcia with whom Dr. George Deligeorgis and Acoustic wave based and innovative Detection Microsystems (Microsystèmes de Détection à ondes Acoustiques et alternatives - MDA) group had collaborated for the initial graphene coated Love wave devices. With his knowledge and expertise on chemical based composites, this research couldn’t go further, since he provided enough evidence to understand the preparation, characterization and morphology of the graphene based solutions under different oxidation levels. Evidently, he has opened the horizon to further control of material parameters
x
to tailor the properties of the second generation of graphene-based Love wave devices. Furthermore, I would like to acknowledge Ms. Veronique Conédéra from Laboratory for Analysis and Architecture of Systems (LAAS - CNRS) for providing high quality Love wave devices. We were working together since day 1 of my thesis and she was the one that firstly introduced me to the world of Drop on demand (DoD) inkjet-printing process electronics; a fact that I am grateful of. For the realization of this thesis I had the chance to collaborate with other laboratories too, within the University of Bordeaux. I would like to acknowledge Associate Prof. Sebastien Bonhommeau and Mr. David Talaga, from the University of Bordeaux – Institute of Molecular Sciences (Institut des Sciences Moléculaires - ISM) for the numerous Raman and Atomic Force Microscopy (AFM) experiments, especially on the highly oxidized graphene Love wave devices. The research conducted on ISM laboratory, was crucial, since it shed light on the electro-mechanical properties of the graphene oxide nano-sheets. Associate Prof. Sebastien Bonhommeau and Mr. David Talaga have provided exceptional experimental premises in a very short amount of time, that is to say, without their scientific contribution and responsiveness, the Physica Status Solidi (PSS) publication would not have been realized. From the University of Bordeaux, I would like to thank the theoretician from the MDA group, Dr. Ollivier Tamarin, for the countless discussions on the acoustic wave theory and mechanisms, analyzing and understanding deeply some crucial elasto-viscosity effects, especially when those are mixed with carbon allotropes / porous materials. It is worth noting, that Dr. Ollivier Tamarin has equally contributed on any scientific research that we published together, besides the fact that we had a limited amount of time to produce, understand and validate the theoretical aspects regarding the analytical modelling of the graphene - coated Love wave devices. Our collaboration has been established during my 3rd year of the Ph.D. and without his rigorous corrections on the manuscript, this thesis would not be fully realized. I was truly honored to work with such a talented scientist. Finally, I would like to thank Jean-Luc Lachaud for assisting in setting up the Vector Network Analyzer (VNA) and installing the permeation tubes in the gas-line characterization room. Further, due to his novel and efficient calibration design kits, as well as his counter-intuitive problem-solving mindset, we were able to collaborate and solve any cell-engineering difficulties and electrical characterization problems. I was happy to work with a person that acquires a MacGyver’s skillset. Last but not least, it would be unfair to forget the significance of Dr. Damien Thuau due to his excellent scientific assistant on the optical interference profiling measurements regarding the thickness of the coated graphene films on the Love wave devices. The presentation of the aforementioned results was realized on IEEE sensors conference in Korea (November of 2015). At that chronological point, his contribution was really essential, since I was attending a five-day conference with a highly research impact within the scientific community of sensory devices. I would like to acknowledge also Dr. Vincent Raimbault, a National Center for Scientific Research (CNRS) fellow, for his numerous scientific discussions and experimental contributions, as well as for his philosophical oriented approaches about science and research life. It would be a shame to forget his impact on this Ph.D., since through his discussions, knowledge and experience, it was later implemented (at the end of my 1st year) a commercial humidity sensor on the gas-line characterization room in order to compare our devices and their efficiency. It might not seem trivial but under his guidance I was able to program the Arduino devices for the humidity sensing, as well as to acquire efficient and fast coding skills regarding the electrical characterization of the Love wave devices. Further, I was truly lucky to have the chance to work with such a distinguished and well-oriented research scientist, since his contributions are highly observable through the outcome of my early published research based on useful measurements regarding the humidity detections. I would like to acknowledge also Ms. Valérie Abel and all the secretaries of the group MDA and IMS,
xi
respectively. Especially, I am really grateful to all of them for all their assistance in administrative stuff during all these years that I spent working in the lab.
During my time in Bordeaux, I took invaluable life lessons - both on a scientific and a personal level. By observing my bosses, not only I learnt how to manage several situations inside a work environment, but I also observed how to actually guide, lead and motivate people; a lesson that would be important in the future of my career. However, those weren't the most important things that I was taught. Through my various experiences inside the lab-life I learnt a really essential lesson; as long as you know where you stand, not only it doesn't matter what people think or say about you, but also that there is nothing and no-one to be afraid of. Associate Prof. Hamida Hallil taught me this lesson personally, and I am grateful to her for that, as true self-confidence is a crucial trait both for a scientist or a leader, in a laboratory or a company, respectively. And usually, this kind of 'self-knowledge' doesn't come easy.
Finally, I would like to thank all my friends, multinational and Greek, inside and outside IMS-laboratory. If I had to name each and every one of you, I would have to write another thesis. I would like to thank all of you, personally, for sharing and having precious time together. Being in a university that gathers people from various nationalities, I was lucky to forge friendships with people from all the edges of the world. And with this, I realized that it is important to draw wisdom from many places. If we take it from only one place, it becomes rigid and stale. But, interacting and understanding the other cultures can help you become whole as a person.
I would like to thank one more time everyone within the faculty for their understanding and for providing me this opportunity.
And, last but not least, according to Niels Bohr ‘an expert is a man who has made all the mistakes which can be made, in a narrow field’; overcoming plenty of difficulties during my Ph.D. in a French educational system forged my personal development for the benefit of all. Also, the presence and support of my friends and family made this experience extremely and truly wonderful. During the period of my thesis I had faced family and personal issues, so it wasn't an easy time, neither for me in Bordeaux, nor for my family in Greece. Thus, I dedicate this thesis to my family. And of course, I would like to thank them for their non-stop support all these years.
Stay hungry, stay foolish Merci beaucoup pour tout, Ioannis »
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Table of Contents
Contents
Contents ... xii
Chapter 1 Introduction and Organization of the dissertation ... xiv
I. Introduction ... 1
I.1 Foreword ... 1
I.1.1 Motivation - Thesis work summary ... 1
I.1.2 Significance of this work ... 2
I.1.3 Organization of the dissertation ... 3
I.2 Literature review – Materials and SAW devices for gas sensing applications ... 5
I.2.1 Introduction - Gas/Vapor sensing applications ... 5
I.2.2 SAW chemical sensors ... 6
I.2.3 Acoustic wave families ... 7
I.2.4 Sensing materials for SAW devices and vapor detections ... 14
I.3 Conclusion ... 23
References or Bibliography (if any) ... 25
Chapter 2 Sensors and Materials Integration ... 34
II.Graphene Oxide sensing material integration in the acoustical platform ... 35
II.1 Introduction ... 35
II.2 Love wave platform fabrication ... 35
II.3 Preparation of graphene oxide solution and its Drop-Casting deposition ... 37
II.3.1 Graphene Oxide solution preparation and its characterization ... 37
II.3.2 Drop - Casting technique for GO deposition ... 39
II.4 Preparation of GO based materials and Inkjet-Printing deposition technique ... 45
II.4.1 Graphene Oxide solution preparation and its characterization ... 45
II.4.2 Inkjet-Printing integration of Graphene Oxide thin films ... 47
II.4.3 Inkjet-Printing characterization of Graphene Oxide thin films ... 49
II.4.4 Electrical characterization of the Inkjet-Printed G-O thin films ... 52
References or Bibliography (if any) ... 56
Chapter 3 Sensor simulation ... 59
III. Theoretical studies ... 60
III.1Introduction ... 60
III.2Modeling of the Love wave sensor ... 60
III.3Analytical simulation modeling ... 63
III.4Numerical (FEM) modeling ... 65
III.5Results and discussion: Application to the numerical and analytical simulations ... 71
References or Bibliography (if any) ... 77
Chapter 4 Vapor detection tests ... 79
IV. Characterization of the GO - Love wave devices under different vapor ... 80
IV.1 Introduction ... 80
IV.2 Gas line experimental setup ... 81
IV.3 Drop – Casted GO sensor performances ... 82
IV.3.1 Detection of humidity ... 82
IV.3.2 Detection of chemical compounds in a gaseous state ... 86
IV.4 Inkjet – Printed GO sensor performances ... 89
IV.4.1 Detection of Ethanol and Toluene Vapors in real time ... 89
xiii
IV.4.3 Detection of humidity ... 97
References or Bibliography (if any) ... 101
Chapter 5 Conclusions and Prospects ... 104
V.The Good, the Bad and the Missing ... 105
V.1 Introduction ... 105
V.2 The Good ... 105
V.3 The Bad and the Missing ... 107
V.4 Finally… ... 108
VI. Appendices (if any) ... 109
VI.1 Author’s contributions ... 109
VI.2 Appendix A - Chapter I: Classification of acoustic wave devices and gas sensing mechanisms ... 110
VI.2.1 A.1 – Classification of acoustic wave devices ... 110
VI.3 Appendix B - Chapter II: ... 112
VI.3.1 B.1 Post-fabrication procedures ... 112
VI.3.2 B.2 – Electrical measurement procedures & Calculations ... 113
VI.4 Appendix C - Chapter III: ... 119
VI.4.1 C.1 Maple analytical equations &calculations: ... 119
VI.4.2 C.2 Numerical calculations – Comsol formulas: ... 122
VI.5 Appendix D - Chapter IV: Gas line experimental conditions and further investigation of the graphene based sensitive material ... 123
VI.5.1 D.1 Vaporizer PUL110 (ppm) and PUL010 (ppb) conditions ... 123
VI.5.2 D.2 Further investigation of the graphene based sensitive material ... 127
I.
Introduction
I.1 Foreword
The past several decades have witnessed a tremendous development of chemical sensors in many fields. Detecting gases and toxic vapors with early warning feature are playing increasingly important roles in many fields, including environmental protection, industrial manufacture, medical diagnosis, and national defense. Meanwhile, sensing materials are of intense significance in promoting the combination properties of gas/vapor sensors, such as sensitivity, selectivity, and stability. Thus, various materials, covering from inorganic semiconductors, metal oxides, and solid electrolytes, to conducting polymers, have been exploited to assemble sensing devices with small sizes, low power consumption, high sensitivity, and long reliability. Among them, nanostructured materials, such as graphene, carbon nanotubes (CNTs), and metal-oxide nanoparticles, are widely used in gas sensing for their excellent responsive characteristics, mature preparation technology, and low cost of mass production, since the traditional metal-oxide technologies are running into enormous challenges as they reach the physical limits of existing silicon-based semiconducting technology. As one of the fascinating materials, graphene has aroused scientists’ great enthusiasms in its synthesis, modification, and applications in many fields since 2004, due to its remarkable overall properties, for instance, single-atom-thick two-dimensional conjugated structures, room-temperature stability, ballistic transport, and large available specific surface areas.
I.1.1
Motivation - Thesis work summary
Consequently, the aim of this Ph.D. is to introduce first the overall interest of Surface Acoustic Wave (SAW) devices based on carbon allotropes, such as CNTs and graphene, especially used for environmental or bio-sensing applications i.e. Volatile Organic Compounds (VOCs) as biomarkers in breath. Further, this work is illustrated with a versatile acoustic wave transducer, functionalized with Graphene Oxide (GO), synthesized for ethanol, toluene, ammonia, carbon dioxide, nitrogen dioxide and humidity detections. The devices were designed, simulated, fabricated and characterized according to the target selection and the aspects of the SAW devices. For example, particular ratios between the length and volume of the deposited graphene on SAW devices were selected in order to detect sub-ppm concentration levels of NH3 and NO2,
respectively. Further, the novel properties of the graphene-based acoustic wave devices were studied and modified according to their optimal detection levels and validated over any further electrical and vapor characterization measurements.
Moreover, the devices were characterized by a Vector Network Analyzer (VNA), Raman spectroscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), performed after each step of the fabrication, attesting that our method has a significant impact to the quality performance of the graphene-based surface acoustic wave devices. We have subsequently employed particular analytical modeling to investigate the electro-mechanical
properties of the GO, as well as to extract the elastoviscosity parameters of several GO layers and their impact on the acoustic devices, theoretically and experimentally. While that, we observed a strong correlation of the results with the number of coatings of the GO solution on the supported SiO2, since the properties of the GO fabricated materials were highly dependent on the
specifically designated thicknesses, themselves intrinsically influenced by the material viscosity and Young’s modulus. It has been previously reported in the literature that Young’s modulus generally decreases with increasing of the GO thickness. Instead, we conclude that the number of the GO layer samples display a compressive internal strain, which does not fully relax after the fabrication process. We attribute this behavior to the uncontrollable large number of the graphene-based oxide sites during the fabrication process, as it has been observed later on the Raman spectra, respectively.
Finally, we have studied the gas sensing characteristics of our devices at room temperature as well as at higher temperatures up to 60 °C. The main reason that the temperature was kept at low levels was to test our devices in a very competitive way based on the current industrial demands, minimizing energy consumption, and/or to overcome some of the latest literature detection levels. The measurements confirmed that some of the detections were efficient based on the graphene devices, and as a result, it is possible to open a new discussion regarding the geometry and the morphology of the very specific GO materials. Some of the device measurements were attributed to a better understanding of the detection mechanisms such as physical or chemical adsorption and further in many cases by distinguishing the adsorption and absorption phenomena. At very low concentration levels of the VOCs, we have observed signatures of few Hz variations for a 100 MHz resonant frequency, but high enough, which implies that further investigation is needed to identify selectivity or specificity levels of certain target analytes. Based on the different geometry and thickness levels, the dominant mechanisms may vary in our samples. At higher concentration levels, the sensitivity showed frequency/temperature-robust results according to the very stable oscillation levels which could be identified as the baseline or initial detection levels of each target analyte, subsequently.
I.1.2
Significance of this work
The significance of this thesis is related to a high-yield fabrication method to obtain functionalized GO-based acoustic wave devices. Importantly, we focus on the scalability of the inkjet-printed fabrication process, as well as its compatibility with the existing SiO2 technologies.
To address several issues and to provide reproducibility and stable sub-ppm detections of the targeted Volatile Organic Compounds (VOCs), we have developed a fabrication scheme based on GO grown by inkjet-printing / Drop on Demand (DoD) deposition method. Different methods were used in a similar way on the acoustic wave devices, thus providing efficient results but not in the scalable route of the industrial demands. One of the most crucial steps of the fabrication process was related to the efficient preparation of GO solutions on the SiO2 substrate to suspend
literature that at this stage, it is very difficult to distinguish the atomic layers by a simple method without collapsing the GO layers together. We have found that apart from this effect, the quality of the oxidation process of GO and the pre-treatment preparation processes on the SiO2 substrates
were essential key parameters to successfully functionalize several graphene-based acoustic wave devices. Only when the quality of the GO solution was highly improved, were we able to achieve remarkable results regarding the VOCs and the water vapor detections, as well as to provide useful information about the vapor adsorption kinetics phenomena.
During this research, several exposure vapors such as NH3, NO2, C2H6O, C7H8 and H2O, were
investigated. The aforementioned compounds are known to affect the environment and human health if an over-exposure occurs. By employing the novel graphene-based materials on the proposed acoustic wave devices, the gas sensing performance was explored for different concentration levels (ppm / sub-ppm monitoring conditions). In each specific application, stringent requirements are demanded from the sensing system, especially for the acoustic sensor itself.
I.1.3
Organization of the dissertation
The objective of this thesis is to investigate the nanostructured materials (e.g. graphenes) in the form of thin films for gas sensing applications. The thesis is primarily devoted to this topic, and is divided as follows:
Chapter I presents the motivation and the author’s contributions based on the current literature and introduces a literature review regarding the SAW devices based on alternative sensing materials and further, highlights the gas sensing properties of the Love wave devices based on carbon allotropes.
Chapter II presents premises of the aforementioned GO nanostructured material and its tailoring properties from the material’s perspective. Further analysis describes material synthesis with particular emphasis on graphene’s oxide characterization methods (SEM, FEM, Raman, AFM, etc.), as well as its electro-mechanical properties based on relative deposition techniques (Drop - Casting, Inkjet - Printing).
Chapter III presents the theoretical and experimental methods used for the graphene oxide sensing layers. Moreover, it analyzes the aspects of FEM and analytical techniques to explore the fabrication of multi-sensory devices.
Chapter IV focuses on the vapor sensing performance of the GO-based acoustic wave sensors regarding the C2H6O, C7H8, NH3 and NO2 detection mechanisms at sub-ppm levels. Notably, RH
mechanisms were proposed on a basis of multi-layer adsorption kinetics. The vapor experimental conditions and calibration systems are also described in this chapter.
I.2 Literature review – Materials and SAW devices for gas
sensing applications
I.2.1
Introduction - Gas/Vapor sensing applications
Nowadays, energy and environmental issues brought about by agriculture, transport and industry [1] have become major challenges strongly influencing the public health and social behavior all over the world. VOCs represent major public concerns due to their widespread use in commercial products (aerosol and adhesives) and in industrial processes [1]. In high concentrations, they are associated with cancer and damage to the central nervous system and are also known to affect developing embryos [2]. For these reasons, gas-vapor sensing applications have earned an excellent deal to answer societal challenges. Gas sensing technologies have emerged due to the common applications within the areas of the high interest like industrial production (e.g., alkane series detection) [1],[3]; automotive applications (e.g., VOC generated emissions from vehicles) [4]-[5]; medical applications (e.g., human modality representatives in a virtue of electronic olfactory systems) [6]-[7]; air quality management (e.g., detection of in-house pollutants based on natural sources) [2]-[8]; environmental studies (e.g., greenhouse emission monitoring) [9]-[10].
Thus, gas emerging applications have been engaged into numerous activities in environmental control and manufacturing, as well as domestic and foreign public health policies. These applications require precise real-time measurements for controlling and monitoring plenty of harmful analytes to increase industrial productivity and safety by narrowing the environmental pollution within limits to maintain World Health Organization (WHO) regulations in agreement with the public health and safety guidelines, annually [11]-[12].
Along similar lines, humidity also causes serious side effects from respiratory problems to exhibiting symptoms of various molds and harmful bacteria [13]. Furthermore, detection and control of environmental humidity has direct consequences in industrial processes [14] and environmental contamination [15].
The lack of embedded detection systems and device selection in the aforementioned purposes can greatly cause difficulties on the evaluation of the effectiveness of environmental policies and also generate similar problems in industry. There are several examples that environmental concerns have been grown exponentially, such as the greenhouse effect, since carbon monoxide emissions have to be monitored annually. It strongly influences the public health and the social behavior all over the world, as reported by WHO and NASA [16]-[17], respectively. Since air pollution affects significantly the life expectancies in a yearly basis evidence [18], there is a high necessity for embedded- advanced and monitoring detection systems for awareness to prevent this environmental threat to reoccur and to ensure that pollution is not beyond the safety limits, thus highlighting the importance of gas sensors. Moreover, the use of flammable gas species such as CH4 and H2 in domestic houses [19], the toxic or irritating-smell gases such as H2S and NH3 used
in industrial processes [20] and the sensing demands for VOCs in food industry [21] reveal that multi-sensory devices are vitally important in everyday life.
Since, gas sensors are important for controlling industrial health and safety, domestic and environmental monitoring, each application places various requirements on the sensor systems, however the aims remain the same: to achieve accurate and stable monitoring of the target analyte in the range of % to parts per trillion (ppt) concentrations [22] depending on the target analyte and the equivalent exposure limits.
I.2.2
SAW chemical sensors
Chemical sensors have been used widely for gas discrimination and quantification in several emerging gas sensing technologies [23]-[24]. Today one of the major outstanding challenges we face on the interaction of the gas species with sensing layer materials [25] is the specified-functionalized materials in terms of structural [26], chemical [27], and morphological attributes [28] that require sophisticated engineer solutions in order to enhance their gas sensing properties [25]-[29]. Many studies [30]-[31] have been realized for various gas sensing technologies, especially based on SAW sensor devices, [32]-[33]. Between them, the fundamental research areas that receive the highest attention have been narrowed to the investigation of different types of SAW designs, novel sensing layers or over-layer functionalization [34]-[35] / immobilization [36] along with industrialization-based techniques. Since the particular demands for gas sensing devices have been analyzed above, a brief classification of SAW device technologies is now given, accompanied by the acoustic wave family, since it is commonly used as a stand-point detection device for environmental applications. Moreover, current research status and recent developments in the field of SAW sensory devices are reported, to provide any sufficient discussions or conclusion regarding the aforementioned problems and solutions.
The main application of SAW devices is as sensors as part of a chemical sensing system [37]. It consists of a target analyte interrogation unit (e.g. sensing layer), the emitter and receiver of the generated and propagated acoustic waves, respectively, and the acoustic path or commonly named as SAW delay line [37]-[38].
In that sense, the type of the acoustic wave devices [37]-[38] is frequently diversified according to the acoustic wave propagation characteristics. The general SAW delay-line principle is based on the emitting and receiving electrodes to generate acoustic waves, as represented in Figure 1-1. Thus, when an alternative electric field is applied to an InterDigital Transducer (IDT), patterned on a piezoelectric surface, it generates acoustic waves (commonly named reverse - piezoelectric effect) [37]. The properties of the piezoelectric material, such as the crystallographic orientation, thickness and material selection determine the family of the acoustic wave sensor and affect the acoustic wave propagation velocity [38]. The piezoelectric substrates that are widely used in the field of acousto-sensors are Quartz (SiO2), Lithium Tantalite (LiTaO3), Lithium
significant role in the material selection of the piezoelectric substrates is the electro-mechanical coupling coefficient (K2) [39]. This is a measure of how effectively a given piezoelectric substrate
can convert an applied electric signal into a mechanical displacement and vice versa [40].
Figure 1-1 Example of a SAW delay-line with IDTs defined on a piezoelectric substrate [37].
In the case of a SAW delay line based on the most renowned Rayleigh wave polarization, the acoustic wave propagation is maintained along the x-axis, which is also the X-crystallographic axis, and the particle displacements occur in the sagittal plane (x-and y- axes on the Figure 1-1), thus offering ascend to an ellipsoidal movement. During the propagation of the generated waves (along the surface), most of the ‘acoustic’ wave energy is focused within a particular depth of the substrate, at the surface.
By principle, the traveling path of the acoustic waves, as appeared in Figure 1-1, is used to maintain the wave propagation characteristics. The traveling acoustic waves are converted finally into an electrical signal by the output-IDTs. The delay-time can be calculated by the following form t = L/v0, where L represents the center-to-center distance between the input and output IDTs
and v0 represents the SAW phase velocity.
Furthermore, the IDTs spatial periodicity λ defines a particular wavelength at which the acoustic waves generated by the adjacent pairs of electrodes will be added in phase. Finally, the essential key properties regarding the acoustic wave characteristics could be included in the number of the finger electrodes, the acoustic aperture, the center-to-center IDTs distance and the transducer periodicity.
I.2.3
Acoustic wave families
Acoustic wave based sensors incorporate plenty of devices according to their multi-variational properties and material characteristics. These sensory devices are commonly based on the diversity of the acoustic wave polarization and propagation mechanisms, determined by the nature of the substrate, the crystallographic orientation, the direction of particle displacement and the sensing material. As described in a previous section, the aforementioned structure of the
IDT-electrodes is used to manipulate the profile of the acoustic waves and their propagation characteristics, since the acoustic waves a) could travel through the bulk of the substrate material, b) could be guided by an over-layer, or could be guided by reflections from multiple surface areas, with respect to the application environmental demands (gas/liquids) [41].
Regarding the polarization of the acoustic waves, three types can be distinguished such as longitudinal, transverse vertical and transverse horizontal. The particle direction of the longitudinal waves or compressional waves should be regarded as parallel to wave propagation direction, whilst the particle displacement of the transverse waves or shear waves is perpendicular to the propagation direction. Furthermore, the acoustic wave devices can be separated into three different categories, such as Bulk Acoustic Wave (BAW), Surface Acoustic Wave (SAW) and Acoustic Plate Mode (APM) devices [42].This classification can be divided into various sub-categories that already exist in the literature [37]-[38], [41], depending on the generation of the acoustic waves and their propagation mode. In particular, the BAW devices are divided into Quartz Crystal Microbalance (QCM) [43] and Film Bulk Acoustic Resonators (FBARs) [44], the SAW family is represented by Rayleigh, Shear-Horizontal Surface Acoustic Wave (SH-SAW) [45], Surface Transverse Wave (STW) [46] and Love wave devices [47] while the APM family includes the Shear-Horizontal Acoustic Plate Mode (SH-APM) [48] and the Flexural Plate Wave (FPW) [49] devices. Precisely, in the first group (BAW devices), the polarization can be longitudinal or transverse, thus it is the volume of the material (substrate) that deforms, allowing the acoustic wave to propagate unguided. The SAW devices can be represented by a Rayleigh wave [50] (longitudinal and transverse), or by the waveform of Bleustein-Gulyaev [51], which is polarized in the transverse horizontal plane. Generally, in SAW devices, the acoustic wave propagates guided or unguided, along the surface of the substrate [52]. When the SAW is unguided and shear polarized, the generated acoustic waves can be represented by the SH-SAW device, whilst the guided and shear polarized acoustic waves corresponding to the guided SH-SAW or Love wave mode devices [52]-[53]. In the third group (APM devices), the wave propagates in the volume of the substrate material, either in the form of two Rayleigh waves (one per side) [45], such as a Lamb wave [54] or in the form of a transverse horizontal wave undergoing reflections at both surfaces of the material plate (SH-APM) [52].
Depending on the classification of the acoustic waves (Figure 1-2), they can be utilized in liquid or gaseous environments or both. The primary norms of the subsequent-based detection systems are ordered by the nature of the acoustic wave propagation characteristics.
Figure 1-2: Acoustic Wave devices can be classified into three main groups depending on the generation of the acoustic waves. Severe sub-groups can be identified for further analysis based on the wave propagation characteristics, as depicted from [37]-[38], [41], [52].
The impact of high demanding performances in environmental applications necessitates an overview perspective of the acoustic wave family devices, according to their generation and propagation properties.
I.2.3.1
QCM
One of the most prototypical devices in the field of acousto-sensors is represented by QCM [52]. Analytically, as illustrated in Figure 1-3, a typical QCM device is based on the oscillation frequency of the vibrating crystal which is sandwiched between two electrodes. The mass sensing mechanism is related to the resonance frequency of the quartz crystal generated by an applied voltage via the electrodes. Therefore, any mass or elasto-viscosity variations of a “sensitive” layer added on the surface of the QCM will report frequency shifts in a coherent manner.
Figure 1-3 Typical representation of the QCM device [55].
I.2.3.2
FBAR
FBAR devices have attained a great deal of scientific research, especially in mass [56], pressure [57] and temperature [58] detection applications. FBAR sensors are closely related with QCM devices in regards of device structure or principle characteristics (Figure 1-4). However, the significant differences are based on the selection of the piezoelectric materials, as well as on the
inherent capabilities of FBAR that operate in higher resonance frequencies (high resolution) in comparison with QCM devices, thus FBAR devices report increased sensitivity detections to any mass or elasto-viscosity variations on the top electrode.
Figure 1-4 Typical representation of the FBAR device[59].
However, a few drawbacks of FBAR devices reported loss in mechanical energy by the presence of liquid environments, as well as high fabrication costs [60]. Hence, by their very nature, these devices do not contribute to current thesis sustainability.
I.2.3.3
Rayleigh and Bleustein-Gulyaev
Rayleigh waves are a type of surface elastic waves, which are an honorable member along the acoustic waves family. Often, they are represented by a seismic surface wave that causes shaking of the Earth’s surface in an elliptical motion and further used to study earthquake mechanisms [61]. At November’s proceedings in 1885 [62], Lord Rayleigh predicted their existence in isotropic solids, where the waves cause particles to maneuver in elliptic displacement in planes normal to the surface and parallel to the wave propagation direction (Figure 1-5). Along similar lines, they have attained a great amount of research interest, since they are responsible for a variety of temperature [63], pressure [64], and vapor [65] sensing applications. Nonetheless, regarding the SAW device performances, especially in liquid environments, Rayleigh devices have reported low sensing performance characteristics, mainly due to generation of compressional waves and high loss in energy confinement [66].
Figure 1-5 Typical representation of the Rayleigh mode SAW device [55].
On the other hand, Bleustein-Gulyaev (BG) waves have shown a particular interest in the scientific literature, since they have been used for several applications in liquid environments. For example, measuring viscosity characteristics in liquids under different pressures could be tedious by applying conventional methods, whereas BG waves have reported great performances [67], since they operate as pure shear horizontal waves. In the presence of viscous liquid, the wave energy is concentrated in the region adjacent to the piezoelectric surface associated with liquids, thus providing to the particles, maneuvres that are in parallel to its surface and normal to the propagation direction.
I.2.3.4
SH-SAW
In general, the SH-SAW devices have attracted a significant amount of attention within the research community, mainly due to high sensitivity performances in liquid environments [68], as well as in bio-detection applications [41]. The primitive conditions that a pure shear horizontal wave can be generated, usually require the commonly reported interdigitated transducers, an acoustic wave delay-line, and a piezoelectric substrate (Figure 1-6). The main principle of these devices being based on the interactions that cause perturbations in the acoustic wave propagation [69] and translated into an electrical signal that can be measured in a simple manner, various applications can be encapsulated under the SH-SAW devices in order to interact with different gaseous [45] or liquid environments [70]. As for other acoustic wave devices, signal attenuation, phase wave velocity and frequency variations are representatives of the measured disturbances caused in the acoustic wave propagation, and thus specific detections could be identified uniquely according to the analyte selection.
Figure 1-6 Typical representation of the SH-SAW device [55].
I.2.3.5
STW
STW represent an alternative way in the acoustic waves family that provides efficiency in plenty of applications by utilizing materials and technology of the existing micro-fabrication technology. Along similar lines, they have gained an extensive amount of research interest as mass or viscosity sensors [71], because they exhibit great performances. As represented in the Figure 1-7, in sensor applications, the STW devices offer the convenience of launching and receiving waves by conventional electrodes that are shaped as parts of an overall grating structure
on the surface of the substrate [71]. In accordance with the illustrated Figure 1-7, the sensing principle of a STW device can be described by using a sensing layer that could react with preselected target analytes that are to be measured. Finally, STW devices can be easily considered as precursors of the Love wave devices, since by changing the metallic grating strip of electrodes to a thin solid film, the wave can be transformed from STW to Love wave [72].
Figure 1-7 Typical representation of STW device [72].
I.2.3.6
Love waves
Guided shear-horizontal waves were firstly described by Augustus Edward Hough Love [53] in his early attempts to explain seismic data that have played a significant role in the scientific literature. His work is represented by numerous contributions in the plurality of the scientific domains, being well-known for its mathematical expeditions in the theory of elasticity. Following his work on the structure of the Earth in Some Problems of Geodynamics, he won the Adams prize in 1911, when he developed a mathematical model of surface waves, known as Love waves.
Figure 1-8 Typical representation of Love wave sensor upon dual delay line device configuration; Love wave propagation direction and particle motions [73], [74], [75].
Besides the theoretical premises of Love waves, they have been reported in many applications for academic [76]-[77] or industrial purposes [71], as well as they can be easily incorporated in sensor devices [35], as shown in the left side of Figure 1-8. Love wave technological nodes are in agreement with the semiconductor fabrication processes of Microelectromechanical systems (MEMS), thus allowing Love wave devices to be integrated in a single chip applications [78], such as lab-on-a-chip devices. In that sense, they have repeatedly shown great performances in
bio-sensing [79] and microfluidic [80] applications, and therefore leading the way towards sensor activities [81].
Furthermore, according to the Love wave principle, the particle motion can be represented by a horizontal line along [0,x2), perpendicular to the propagation direction along [0,x1) and parallel to
the surface, as represented in the right side of the Figure 1-8. From the wave’s perspective, the particle motion can be decreased to a particular point of the sagittal plane due to the depth-dependent restrictions that should be followed to permit the Love wave excitation. According to the surface boundary conditions, the shear bulk wave velocity of an over-layer (i.e. SiO2) must be
lower than the substrate’s shear bulk wave velocity (i.e. AT-cut quartz), in order to maintain the critical angle so that shear horizontal reverberations can be totally trapped, and therefore allow the generation of Love waves [82]. Moreover, the Love waves decay exponentially with the depth, since their energy confinement is maintained to the over-layer surface. Along similar lines, acoustic wave perturbations can induce variations in the wave propagation characteristics, especially caused by gravimetric [83] or viscoelastic [84] effects, thus a resulting attenuation in the output signal (Figure 1-8, wave’s representation [75]).
However, unlike many previous studies [85]–[87] of the Love wave sensors, this research only examined the Love wave device’s performances, effects and aspects under vapor environments [88]-[89], especially using graphene-sensitive [90]-[91] layers from the carbon’s materials family. The current research of Love wave devices has therefore highlighted the effectiveness of the aforementioned bared and coated sensors, as well as reported results in the literature of the acoustic wave sensors. It shed light on tedious, complex and often controversial [90] concepts regarding the viscoelastic properties of graphene-based Love wave devices, and further studied the variability emerged across humid vapor detections [92] towards multi- and monolayer adsorption kinetics (i.e. modified Brunauer-Emmett-Teller method [92]).
I.2.3.7
SH-APM
A different sub-type of the acoustic waves family, similarly well-known for its device’s performances, is the commonly reported SH-APM devices. The term APM, is related to the wave excitation in a plate. The plate mode differs according to the propagation direction of the waves, and the particle motion inside the plate. The most attractive category of the acoustic plate modes is demonstrated by the SH-APM, especially due to the sensing performances and the non-coupling effect in liquid environment applications [41], [93]. As commonly reported for the acoustic devices, similar characteristics may apply to the SH-APM devices (wave perturbation, detection method, etc.) regarding the piezoelectric material and the IDTs, in which plenty of sophisticated configurations can prevent any contact of the electrodes with the liquid sensing environments.
I.2.3.8
FPW
FPW are sauntered as the ‘medium’ rendition towards SAW devices. The basic premises of acoustic’s theory are related with the advanced fabrication practices of the acoustic wave devices, that employ an excitation of the piezoelectric substrate through the IDTs and confine the acoustic wave energy near to its surface. On these grounds, when the thickness of the piezoelectric substrate is very thin, and precisely lower than the wavelength of the acoustic wave, then Lamb waves can be generated, or commonly reported as Flexural Plate Waves. The significance of the Lamb wave devices occurs mainly in liquid sensing applications [94]; however they have shown a particular interest in recent literature [95], especially used as humidity sensors [96], thus overpassing challenges that are related to complex fabrication techniques.
I.2.4
Sensing materials for SAW devices and vapor detections
I.2.4.1
Overview of materials for SAW-based gas sensors
Besides the fact that WHO regulations are frequently changed, especially under air quality corrigendum [11],[97] the International Health Regulations (IHR) suggest, prevent and respond to severe chemical, physical or biological health risks. Subsequently, peerless challenges have emerged in the fields of chemical and bio- sensors, commencing from the design of specific material characteristics to the ability to solely detect and identify targeted analytes, selectively.
Aside from all these functional or chemical prerequisites, the sensitive and/or selective materials must be suitable with SAW devices, in terms of material integration and adhesion-related processes, thus providing efficient and low-cost devices towards global production. Many SAW devices are being referred into the spin-coating, spray-coating, inkjet-printing, sputtering, drop-casting and electro-spraying techniques, according to the nature and objectives of the sensing demands.
Nowadays, a step forward into the new era of SAW devices and engineering technology, there is plenty of science-based evidence to conquer the aforementioned challenges, since the scientific and technological wherewithal is supplied constantly by a variety of sophisticated materials, especially for vapor sensing applications. In particular carbons, metals, polymers, hybrid and meta- related materials have been reported for recognizing particular vapors and analytes, especially under functionalization of the sensing layer(s) or surface tailoring compliance. Selectivity is contingent upon a definition of ‘ability of the chemical-SAW sensor to respond solely to a group of analytes or even specifically to a single analyte’. Furthermore, these novel materials are continuously evolved through the ‘Darwinism’ material selection, which is put forward under the guise of its sensing and recognition effectiveness; nonetheless they aren’t followed by the publish or perish norm philosophy, since most of the time they are adopted by industry.
Therefore, as illustrated in Figure 1-9, plenty of materials have been reported for vapor sensing applications. The foregoing discussion implies that selective responses and vapor recognitions can
be categorized (Table 1.1) according to the nature of the sensing material and device integration criteria consistent with the available SAW sensor’s performances.
Figure 1-9 An illustrative representation of the material family, recently used as vapor sensing layers on SAW devices. The material classification is descended and inspired by [98] to perceive larger material diversification and a plurality conception, which can be conceived and delivered in an interesting and carbon related manner for SAW devices.
Material family Sensing
(mono)-layers Target Vapors
Sensitivity (Hz/ppm) or LOD (ppm) Refs Metal and Metal oxide structures WO3 C2H6O / H2O 238 Hz/ppm, LOD 10 ppm for C2H6O at 300 °C; [99] Pd electrodes H2 6-8 Hz/ppm, LOD 100 ppm for H2/N2 at 250 °C; [100]
Indium Tin Oxide (ITO) NO2
51.5 °/ppm, LOD 5 ppm for NO2/N2 at 240°C; [101] Pt/ZnO nanoparticles H2 5.5 Hz/ppm, LOD 2500 ppm for H2/N2 at Room Temperature [RT]; [102] In2O3 H2 8.8 Hz/ppm, LOD 100 ppm for H2/N2 at RT; [103]
ZnO, SnO2, TeO2, TiO2 NH3
ZnO (40 nm) ~3 Hz/ppm, LOD 400 ppm for dry NH3/N2 at RT; [104] Carbon based / Carbon hybrid materials Graphene-like nano-sheets (incomplete GO reduction) H2 / CO 0.6 Hz/ppm (H2), LOD 600 ppm for H2/air at RT; 8.5 Hz/ppm (CO), LOD 125 ppm for CO/air at RT; [105]
Graphene Oxide
(Drop-Casted whole device) H2O
~53 kHz / % RH, LOD 0.5 %RH for H2O/N2 at RT;
MWCNTs/Nafion
nanofibers H2O
~400 kHz/%RH, RH detection
precision 0.5 % RH at RT; [107]
Graphene Oxide (Inkjet-Printed between IDTs)
C2H6O / C7H8 / H2O 30 Hz/ppm (C2H6O), LOD 100 ppm for C2H6O/N2 at RT; 24 Hz/ppm (C7H8), LOD 100 ppm for C7H8/N2 at RT; 2.4 kHz/%RH, RH monolayer ~173 kHz for H2O/N2 at RT; [92]
Graphene Oxide
(Drop-Casted between IDTs) C2H6O / H2O
112 Hz/ppm (C2H6O), LOD 100
ppm for C2H6O/N2 at RT;
6.4 kHz/%RH, LOD ~2%RH for H2Oat RT;
[89]
Graphene/PANI NO ~320 Hz/ppm, LOD 0.5 ppm for
NO/air at RT; [108] Graphene (Liquid phase
exfoliated graphene / defected Graphene Oxide) NO2 25 Hz/ppm, LOD ~0.5 ppm for NO2/air at RT; [109] Graphene-nickel (Ni)-L-alanine CO2, Ar, O2, C2H6O 2.1 Hz/ppm (0-2000 ppm of CO2/N2), LOD 200 ppm for CO2/N2 at RT; [110] SWCNTs / MWCNTs C2H6O / C4H8O2 / C7H8 6.89 kHz/ppm (SWCNTs), LOD 1.3 ppm for C2H6O/N2 at RT; 5.45 kHz/ppm (SWCNTs), LOD 1.6 ppm for C4H8O2/N2 at RT; 7.47 kHz/ppm (SWCNTs), LOD 1.2 ppm for C7H8/N2 at RT; [111] MWCNTs-COOH- Poly(n,n-dimethylamino propylsilsesquioxane) [SXNR] C2H6O 9 kHz / ppm (oxidized MWCNTs or MWCNTs-COOH-SXNR), for C2H6O/N2 at RT; [112] 10 monolayers of SWCNTs in CdA LB / 30 monolayers of SWCNTs-in-CdA LB NO2 / NH3 / H2 3.3×10−2 °/ppm (SWCNT-in-CdA LB), LOD 0.16 ppm (30 monolayers of SWCNT-LB) for NO2/air at RT; 4.7×10−4 °/ppm (SWCNT-in-CdA LB - 10), LOD 33 ppm for NH3/air
at RT;
1.8×10−5 °/ppm (SWCNT-in-CdA LB - 10), LOD 330 ppm for H2/air
at RT;
Cu nanoparticles-SWCNTs
H2S / H2 / C2H6O
/ C3H6O
~2.6 kHz / ppm for H2S/Air, LOD
5 ppm at RT and selectivity for H2S/Air at 175°C;
[114]
Polyepichlorohydrin [PECH] with different %
MWCNTs / Polyetherurethane [PEUT] with different %
MWCNTs C8H18 / C7H8 ~1Hz/ppm (PECH with 5% of MWCNTs), LOD 9.2 ppm for C8H18/air at RT; ~4.4 Hz / ppm (PECH with 5% of MWCNTs), LOD 1.7 ppm for C7H8/air at RT; [115] Polyisobutylene (PIB) mixture with MWCNTs C8H18 / C7H8 ~8.1 Hz / ppm (PIB with 2% of MWCNTs), LOD 25 ppm for C8H18/air at RT; ~3.3 Hz / ppm (PIB with 2% of MWCNTs), LOD 25 ppm for C7H8/air at RT; [116] Polyallylamine [PAA]-amino-CNTs / Polyethyleneimine [PEI]-amino-CNTs CO2 ~0.3 Hz / ppm (PEI-amino CNTs), LOD 500 ppm (PAA-amino-CNTs) for CO2/air at RT; [117] Polymers Teflon AF2400 CO2 ~2.1 ° / ppm, LOD 75 ppm for CO2/N2 at RT; [101] Polyepichlorohydrine [PECH] C7H8
2 Hz/ppm (PECH with ZnO device - 10% O2), LOD 25 ppm for
C7H8/air at RT;
[118]
[PIB], [PECH], [PEUT] C8H18 / C7H8 / C4H8O
4 Hz /ppm (PEUT - S3 device) for C8H18/air at RT, LOD –
probabilistic neural network (PNN) classification of different gas concentrations (C8H18 / C7H8 /
C4H8O) with a 100% success rate;
[119] Polymethyl[3-(2-hydroxy) phenyl]siloxane [PMPS] C3H9O3P (DMMP) 3 kHz/ppm (PMPS), LOD 5 ppm for C3H9O3P/N2 at RT; [120]
Poly(ophenylenediamine) - Molecularly Imprinted Polymers C3H9O3P (DMMP) 487 Hz/ppm, LOD 0.1 ppm for C3H9O3P/N2 at RT; [121]
[PECH], [PEI], [PIB]
CH2Cl2 (DCM) / C4H8O2 (EtOAc) / C3H9O3P (DMMP) / C4H10FO2P (GB) 649 Hz/ppm (GB), LOD 9.24x10-3 ppm for C4H10FO2P/N2 at RT; [122] Supramolecular materials Tert-butylcalix[4] arene C2Cl4 (Tetrachloroethyl ene) 3.5 Hz/ppm, LOD 50 ppm for C2Cl4/air at RT; [123]
Calix[4] - Calix[6] arenes C8H10 / C7H8 / C2Cl4 6 Hz/ppm (P-tert-butylcalix[4]arene), LOD 20 ppm for C8H10/air at RT; [124] (3) Calixarene layers - Poly(diallyldimethylamm
onium chloride) [PDDA]
C2Cl4 / C2HCl3 / CHCl3 ~8.7 Hz/ppm (C2Cl4/air) ~4.5 Hz/ppm (C2HCl3/air) ~3.7 Hz/ppm (CHCl3/air) for (3)-Calixarene layers-PDDA, at 15°C; [125] Self-assembled monolayers (SAMs) Self-assembled Calixarene Derivatives C3H9O3P (DMMP) / C7H17O3P (DIMP) ~4.4 kHz/ppm (DMMP - 5mg/m3), LOD 19.7x10-3 ppm (0.1 mg/m3) for C3H9O3P/N2 at 28°C; [126] Siponate DS-10 / PolyEthyleneGlycol [PEG1000] C6 alcohols / C7H14O2 (n-amyl acetates)
-, LOD < 1-2 ppm for odorants/air
at RT; [127] Self-assembled derivatives (lipopolymeric layers) C4H10O (n-butanol) ~145 Hz/ppm (Quartz ball-SAW device), LOD 29x10-3 ppm for
C4H10O/air at RT; [128] Nano- modifiers / Nano-composites SiO2/Si - MWCNTs in Polyethylenimine [PEI] C2H6O / CH3OH / C7H8 1.19 Hz/ppm (MWCNTs-PEI), LOD 15.2 ppm (Si/SiO2-PEI) for
C2H6O/air at RT;
1.14 Hz/ppm (MWCNTs-PEI), LOD 16.7 ppm (Si/SiO2-PEI) for
CH3OH/air at RT;
1.23 Hz/ppm (MWCNTs-PEI), LOD 13.8 ppm (Si/SiO2-PEI) for
C7H8/air at RT; [129] Surface-modified diamond nanoparticles C7H6N2O4 (DNT) / C3H9O3P 80 kHz/ppm (DNPs), LOD 0.2 ppm for C7H6N2O4/air at RT; 4 kHz/ppm (DNPs), LOD 0.5 ppm [130]
[DNPs] (DMMP) / NH3 for C3H9O3P/air at RT;
70 kHz/ppm (DNPs), LOD 30 ppm for NH3/air at RT;
Langmuir–Blodgett (LB) nanolayers DA, CA,
DA-CA
C2H6O / CH3OH
16x10-3 Hz/ppm (DA - porous), for
C2H6O/air at 35°C;
30x10-3 Hz/ppm (DA - porous), for
CH3OH/air at 35°C; [131] Laser-Deposited [LD] nanostructured of polyethylenimine [PEI] C2H6O / CH3OH / C7H8 5, 4.5 and 5.4 Hz/ppm (LD-PEI), LOD 6, 6.7 and 5.6 ppm for
C2H6O/air, CH3OH/air and
C7H8/air, respectively at RT; [132] Polyaniline/In2O3 Nanofiber Composites H2 1.1 Hz/ppm, LOD 600 ppm for H2/air at RT; [133] ZnO nanorods H2 183 Hz/ppm, LOD 500 ppm for H2/air at 265°C; [134]
Table 1.1 Classification of alternative materials according to their nature, device integration, vapor selection and sensing properties criteria for SAW devices, adapted from [98].
From the outset of gas sensing applications, devices regarding the vapor detection [99] or gas [102]-[103] sensing applications have widely used metal oxide nanostructures as an enhancing layer, especially due to sensitivity and selectivity performances [99]-[104]. However, in many cases individual devices were able to perform, particularly at elevated temperature conditions [99]-[101]. The elevation of the temperature is usually based on the reaction heat between the gas species and the sensing layer that leads to the necessity for overcoming challenges like low energy consumption and sustainable gas detection devices at room temperature.
On the other hand, polymer based sensitive layers have been selected very often for the exploration of detection materials in the toolbox of SAW gas sensors. Polymer coated devices provide an attractive way to form the required shape and thickness of the sensing layer, which has proved some reversibility for gas sensor applications. These versatile polymers can be used at ambient temperatures [101], [118]-[122], which is a great advantage for vapor sensing applications due to lower energy consumption. Notwithstanding the easy fabrication processes of polymer coated structures, plenty of sophisticated polymer designs offer a route to vapor sensing materials with a good selectivity to certain analytes, such as for DMMP detections [120]-[122] for example, which insures high performance of the SAW sensors. However, a few polymer disadvantages include sensitivity to oxidation and humidity mechanisms or require a marginally complicate polymer preparation.
Supramolecular structures are large molecules formed by grouping or bonding smaller molecules together. They are usually generated through the developing of macro-molecules that are not covalently bonded, and thus form a desired shape or functionality. By taking advantage of the formed characteristics regarding the host (supramolecular) – guest (target analyte) interactions, particular structures can be obtained for specific sensing purposes. One great example such as calixarene sensing layers provides the aspect of molecular receptors for different analytes and exhibits high affinity and great selectivity towards vapor detections [123]-[125]. However, a common drawback that is affected by calixarene layers is that the supramolecular coated SAW devices tend to report cross sensitivities [125] towards Tetrachloroethylene (C2Cl4), Trichloroethylene (C2HCl3) and Chloroform (CHCl3) volatile
compounds, respectively.
Self-assembled monolayers (SAMs) represent one very useful category of sensing layers for SAW devices. Since SAMs are molecular assemblies formed on surfaces, as well as organized into more or less large ordered domains, they can be efficiently used for specific analytes detection. There are plenty of interesting works in the literature [126]-[128] regarding the detection of vapor compounds, however one of the greatest assets in the field of SAW devices has been realized by calixarene derivatives [126] used as self-assembly molecular imprinted film, and thus showing high sensitivity detections of volatile agents like DMMP.
Nano-composites offer the opportunity to build certain blocks by a mixture of materials in the nanoscale arena in order to generate materials with novel properties used as sensing layers, especially in SAW devices. The idea of a mixture-based nanocomposites can be illustrated by the addition of SiO2/Si nanoparticles and multi-wall carbon nanotubes (MWCNTs) embedded in
polyethylenimine (PEI) [129], which has proved to be extremely useful for the detection of VOCs such as C2H6O (ethanol), CH3OH (methanol) or C7H8 (toluene).
Along similar lines, nano-modifiers or hybrid-based structures can be used as offered materials through a matrix for the preparation of sensing coatings on SAW devices. Improvements with high affinity towards specific compounds [130] such as C7H6N2O4 (DNT),
C3H9O3P (DMMP) and NH3 have shown a great amount of interest in the literature, however for
both cases (nano-composites and nano-modifiers) specificity still remains an issue [129], [130].
I.2.4.2
Carbon-allotropes for SAW devices
As a preface, it is worth mentioning that the main aspects of this sub-chapter are focused on the graphene’s material family and carbon related materials (graphene composites, defected graphene oxide, CNTs, etc.) used as sensing layers in SAW devices. This field has shown significant amount of interest by many scientists and engineers and it is constantly updated with exceptional impacts frequently, hence listing every single update would feel like a quota or Sisyphous from the ancient myths.
Figure 1-10 Crystal structures of the different allotropes of carbon. Three-dimensional diamond and graphite (3D); two-dimensional graphene (2D); one-dimensional nanotubes (1D); and zero-dimensional buckyballs (0D). (Adapted from [135]).
Carbon is an interesting element due to its capability to form variety of allotropes which are 3D diamond and graphite, 2D graphene, 1D carbon nanotubes, and 0D fullerenes [135].
As illustrated in Figure 1-10, plenty of carbon allotropes have been investigated by the research community, since they have shown a great amount of interest in many applications including sensory devices. A carbon atom can form various types of allotropes. In 3D structures, diamond and graphite are the allotropes of carbon. Carbon also forms low-dimensional (2D, 1D or 0D) allotropes collectively known as carbon nanomaterials. Examples of such nanomaterials are 1D carbon nanotubes (CNTs) and 0D fullerenes [135]. The electron configuration of carbon goes as 1s2, 2s2 and 2p2, respectively. The four valence electrons take part in the chemical bonds, whereas the hybridization provides the capability of single, double and triple bonds. Due to the small energy difference between the 2s and 2p (2px, 2py and 2pz) orbitals, their wave functions
could generate novel orbitals in the form of sp, sp2 or sp3, depending on the number of p atomic orbitals participating in the mixing with the s atomic orbital [136]. For example, in the list of carbon nanomaterials, graphene is known as 2D single layer form of graphite. It is famously known for its interleaving sp2 bonds (trigonal planar at 120°) within the hexagonal lattice, since they are stronger that the sp3 bonds of diamond (tetrahedral at 109.5°), thus highlighting graphene as a very strong material. Last but not least, graphene has been used for many years due to its remarkable behavior regarding the mechanical and electrical properties, and has been actively investigated also in sensory device applications [105]-[110].
Nowadays, graphene-based communities are mainly focused on the synthesis of high-quality nanoscale materials for large-scale production, since it is still remaining one of the biggest technological challenges. Focusing on extraordinary materials, processability played a significant role in the fabrication of graphene. Moreover, nanotechnology emerged to develop plenty of solutions for graphene implementation by using sophisticated technological processes in order to achieve inexpensive and low power devices, and later on to propose its effectiveness as an alternative to silicon-based technology that involves graphene-rigorous, and therefore adaptive micro-electronic strategies [137].
To approach the synthesis of state-of-the-art carbon based nano-composites, scientists and engineers have categorized two following groups, the “Top - Down” and the “Bottom - Up”. Each group is related to the scale and the processing method of the graphene-based material. The “Bottom - Up” approach consists of a molecular carbon precursor that is mainly used in standard
